1. Field of the Invention
The present invention relates to a piezoelectric crystal material, and more particularly to a piezoelectric crystal material of gallium phosphate (GaPO4) and a piezoelectric resonator, that is, a piezoelectric unit, having such a piezoelectric crystal material.
2. Description of the Related Art
Crystals having a piezoelectric effect are widely used in electronic components such as piezoelectric resonators, for example. Various crystals having a piezoelectric effect are known in the art. One of the known crystals is gallium phosphate crystal. Gallium phosphate, which is also referred as gallium orthophosphate, has excellent properties in that it does not cause a crystal phase transition upon a temperature rise and can be used as a piezoelectric crystal member in a wide temperature range up to about 900° C. The use of gallium phosphate as a piezoelectric sensor material is also promising because it has a larger electromechanical coupling coefficient than quartz crystal.
Gallium phosphate has a crystalline structure that is homeotypic to quartz crystal. Based on the crystalline structure of gallium phosphate, three axes, X, Y, Z, that are perpendicular to each other are crystallographically defined for gallium phosphate crystals. If a gallium phosphate crystal is used as a piezoelectric resonator or a piezoelectric unit, then, as shown in FIG. 1, plate 1 having an X-Z plane as a principal surface of the gallium phosphate crystal is considered, and the use of rotated Y-plate 2 having a principal plane parallel to a Y-Z′ plane, which is obtained by rotating plate 1 about an X-axis in a-Y-axis direction through a certain angle α, is taken into consideration. A Z′-axis is an axis that is obtained by rotating a Z-axis about the X-axis through the angle α. Although, as with a piezoelectric quartz crystal, the gallium phosphate crystal can have various oscillation modes as with a piezoelectric quartz crystal, the vibration mode of such a rotated Y-plate is a thickness shear vibration mode.
Rotated Y-plate 2 is cut from a gallium phosphate crystal, and shaped into a thin-plate resonator piece. Excitation electrodes are formed the opposite surfaces of the thin-plate resonator piece, and a voltage is applied between both excitation electrodes to excite piezoelectric vibrations in the thin-plate resonator piece. The resonator comprising the rotated Y-plate of gallium phosphate has a frequency vs. temperature characteristic curve which is an upwardly convex quadratic function curve representing its resonant frequency, as shown in FIG. 2. A peak temperature T0 of the frequency vs. temperature characteristic curve, i.e., a temperature at which the resonant frequency is maximum on the frequency vs. temperature characteristic curve, varies depending on the rotational angle a which indicates how much the Z-axis has been rotated about the X-axis to produce the Z′-axis of the rotated Y-plate. With piezoelectric resonators of gallium phosphate, the peak temperature T0 may be made higher than the normal temperature (25° C.) by changing the rotational angle α. For example, a piezoelectric resonator which has a peak temperature T0 of 100° C., for example, can be realized. This indicates that it is possible to provide a piezoelectric resonator whose resonant frequency is stable near the high peak temperature T0, and a piezoelectric resonator of gallium phosphate can advantageously be used as a reference frequency source in oscillators for use in high-temperature environments.
However, if attempts are made to reduce the plate surface area of a thin-plate resonator piece shaped from a rotated Y-plate of gallium phosphate for producing practical piezoelectric resonators, then the basis of a thickness shear vibration mode which requires the plate surface area to be sufficiently large with respect to the thickness is liable to be lost. If the plate surface area is not sufficiently large, then the piezoelectric resonator generates strong unwanted responses (i.e., auxiliary vibrations) in addition to the objective principal vibrations, and the intensive auxiliary vibrations cause the frequency vs. temperature characteristic curve of the piezoelectric resonator to deviate from a smooth quadratic function curve, and also change the peak temperature. If a piezoelectric resonator having a thickness shear vibration mode comprises a piezoelectric quartz crystal, then Japanese laid-open patent publication No. 51-97394 (JP, 51-97394, A) discloses that the plate surface area has to be sufficiently large with respect to the thickness.
For reducing the size of the resonator of the type described above, it is the general practice to make the resonator slender in the direction of the displacement of thickness shear vibrations, i.e., to make the resonator slender in the shape of a rectangular shape that is elongate in the X-axis direction. However, the unwanted responses that are generated as described above make it difficult to select appropriate resonator configurations, resulting in difficulty to reduce the resonator size.